Direct averaging is often used to improve measurement accuracy in measurement instruments, such as oscilloscopes. By taking the average of multiple signals (or averaging multiple samples acquired from a single signal), the signal-to-noise ratio (SNR) of the result can be increased, since non-repeating noise and distortion are averaged out. But, when the signals have jitter, the averaged result may be distorted. When the signals are averaged, this jitter causes higher-frequency portions of the result to be attenuated more than the rest of the result. This can typically be seen as slower rising edges in the averaged result.
Often, measurement instruments may introduce jitter into the signals that they are measuring. For example, trigger jitter in real-time oscilloscopes introduces jitter into samples acquired by the scope. Thus, these instruments may not be able to make measurements as accurately as more expensive devices, even when averaging is used. Thus, there is a need for improved averaging techniques to minimize the effect that jitter has on the averaged result.
Improved averaging techniques could be useful for a number of different signal processing applications, and might allow instruments with acquisition jitter to replace more expensive instruments. For example, in one embodiment, improved averaging techniques could allow real-time oscilloscopes to measure S-parameters of a device under test.
In one example, the disclosed techniques might allow a real-time oscilloscope to measure S-parameters without needing additional instruments such as a vector network analyzer (VNA). As bit rates increase, high speed serial data link simulation and measurements increasingly need to use S parameters when modeling components in the data link. For example, to fully characterize and simulate the serial data link 100 shown in FIG. 1, the output impedance (represented by reflection coefficient S22) of the transmitter (Tx) 105, input impedance (represented by reflection coefficient S11) of the receiver (Rx) 115, and the full S parameters (S11, S12, S21, and S22) of the channel 110 are all needed.
Traditionally, a vector network analyzer (VNA) or a time-domain reflectometry (TDR) system with a sampling oscilloscope is needed to measure these types of S parameters for two-port or multi-port network characterization. These specialized instruments are often expensive, and are not widely available. In contrast, real-time oscilloscopes are commonly used to debug, test, and measure high speed serial data links. It would be convenient to use real-time oscilloscopes to measure the S parameters of a data link.
Unfortunately, while some previous methods allow a real-time oscilloscope to measure S parameters or related functions, they do not enable the scope to take accurate enough measurements to eliminate the need for additional VNA or sampling oscilloscope-based TDR solutions. For example, one prior art solution by Agilent (described in U.S. patent application Ser. No. 13/247,568 (“the '568 application”)) uses precision probes to measure probe impedance and transfer functions for a Device Under Test (DUT). These measurements may then be used to create embed or deembed filters to compensate for the measured system characteristics. But the transfer functions measured using this method do not provide accurate delay information for the DUT. For example, a longer high quality cable may have the same magnitude loss as a shorter but lower-quality cable. But these two cables have very different group delay characteristics. Because the method disclosed in the '568 application does not measure accurate delay information, it is not accurate enough to determine which type of cable is being used.
U.S. patent application Ser. No. 14/673,747 (“the '747 application”) does describe a method of measuring full S parameters using a real-time scope, along with a signal generator and a power divider. But the method disclosed in the '747 application is still prone to measurement errors due to trigger jitter that is inherent in real-time scopes.
As discussed above, accuracy of real-time scopes can be improved by using averaging. But real-time scopes have trigger jitter, which causes the higher-frequency portions of the signal to be attenuated more than the rest of the signal. This can typically be seen as slower rising edges in the measured signal. Because prior art averaging solutions do not address this attenuation, they do not enable real time oscilloscopes to make measurements as accurately as other instruments such as VNAs or sampling scopes.
In addition, when a repeating data pattern is averaged, the patterns must all be aligned. Traditionally, the patterns are aligned based on edge crossings, or by using cross-correlation. But noise in the patterns can distort the edge crossings, causing a loss of accuracy in edge-based methods. And cross-correlation is computationally expensive.
Thus, there is a need for improved averaging techniques to take more accurate signal measurements.